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points, draw lines from end to end of the prism B, as shown in No. 2. The points where these lines cut the corresponding vertical ones drawn through points 4 4', 5 5', 6 6', in No. 1, will be points in the lines of penetration of the two prisms. On drawing lines through these points, they will be found to be straight ones, crossing each other at the point 2, where the two frontmost edges of the prisms intersect. Although these lines might have been at once obtained by joining the points a d, b c, in No. 2, by straight lines, it is advisable that the student should be able to prove that they become straight, rather than take the matter for granted. If the axis of one of such equal prisms be inclined to the VP, 150 FIRST PRINCIPLES OF MECHANICAL AND ENGINEERING DRAWING 151 the other remaining parallel to it as before, then the lines of penetration will be as shown in No. 4 ; obtained by first finding the elevation of the prisms as if they were entire, and then projecting over from No. 3 which is No. 1 with the axis of the prism B at the desired inclination to the VP to the corresponding edges in No. 4, the points where the edges of B that will be seen in elevation intersect those of the prism A. The right lines joining these points as shown in No. 4 are then the visible lines of penetration of the two prisms. For the next problem a case is taken where, although the axes of both prisms are still in the same plane, yet neither of them is vertical or horizontal. Problem 69 (Fig. 173). Two equal square prisms have their axes in one and the same plane, and intersect each other at the middle of their